The United States on Monday called on Europe to contribute more troops to United Nations peacekeeping missions and announced that President Barack Obama would hold a summit of world leaders in New York in September to win new commitments.

The U.S. ambassador to the United Nations, Samantha Power, said that two decades ago some 25,000 European troops served as peacekeepers, but that number has fallen to fewer than 6,000, or less than 7 percent of the U.N.'s 90,000 peacekeepers.

There are currently 16 U.N. peacekeeping operations around the world, more than half in Africa. The United States pays for more than 28 percent of the peacekeeping, whose costs total than $2.5 billion this year alone.

"Obviously it is not the job of the United States - or any government - to tell European countries how to maintain peace and security. But it is essential that each of us does our fair share," Power told a Friends of Europe think-tank event in Brussels on Monday.

Power said Obama would convene a summit of world leaders on the sidelines of the U.N. General Assembly in September to "help catalyze a wave of new commitments" to U.N. peacekeeping.

She said that since European countries have drawn down their number of troops in Afghanistan to 2,000 from 35,000 four years ago, those freed-up soldiers could make a difference elsewhere.

Greater participation by European troops would also improve standards and modernize peacekeeping missions, Power said.

Power also said that while Washington and European allies had committed to dedicating a minimum of 2 percent of their countries' respective GDPs to defense, only two allies were meeting that benchmark, while others were cutting and not increasing military spending.

"Given the threats that exist around the world, this is deeply concerning," Power said.

The 28-nation EU has battle groups manned on a rotational basis and meant to be available as a rapid reaction force, but they have never been used in a crisis.

"That is regrettable, given the many worthy causes," Power said. "These rotations tie up thousands of troops, who might otherwise be deployed in the field to save lives."

According to the U.N.'s website, the United States contributes 126 troops, police and advisers to peacekeeping missions out of a total of more than 104,000 individuals.